Segfault in parsing char* into tokens

Question

I'm trying to parse a string around an arbitrary index. In my simplest test program I could come up with I have an input string I read the input into and then do memcpy to parse the string.

For testing this I am typing in "this text" as input. readInput is a function where I just have it calling getline(&input, &size, stdnin) and return the input pointer.

int main(){
    char *input;
    input = readInput();

    int parseAround = 4;
    char *token1;
    char *token2;

    memcpy(token1, inputBuffer, 4);
    printf("token: %s\n", token1); //prints "this"

    memcpy(token1, inputBuffer + (parseAround+1), 4);
    //when changed to memcpy(token2,...); segfaults
    printf("token: %s\n", token1); //prints "text"


    free(input);
    return 0;
}

However when I change the second memcpy to use token2 rather than token1, I get a segmentation fault. Why is this?

Neither `token1` noir `token2` is initialized ... they point to random places. Thus when you `memcpy` to them, you are copying to a random locaion, and you segfault. — NickStoughton, Nov 23 '16 at 21:38
`memcpy(token1, inputBuffer, 4);` has no memory allocated to `token`. — Weather Vane, Nov 23 '16 at 21:38

Chirality · Accepted Answer · 2016-11-23T21:45:38.227

You most likely need to allocate memory for token1.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(){
    char *input = NULL;

    size_t len = 0;
    ssize_t read;

    /* Read in the line "Hello world" */
    read = getline(&input, &len, stdin);
    printf("Retrieved line of length %zu :\n", read);
    printf("%s", input);

    /* Allocate memory for both parts of the string */
    char *token1 = malloc((read+1) * sizeof(char *));
    char *token2 = malloc((read+1) * sizeof(char *));

    memcpy(token1, input, 6);
    printf("token: %s\n", token1); //prints "Hello"

    memcpy(token2, (input+6), 5);
    printf("token: %s\n", token2); //prints "world"

    free(input);
    return 0;
}

Read in the line, allocate memory for each string part, and then copy the part you want into your s

`getline` returns the number of characters read, but not including the terminating null byte ('\0'). So you should add one to the space allocated with malloc() to hold that string terminator. — NickStoughton, Nov 23 '16 at 21:42

Segfault in parsing char* into tokens

1 Answers1